Expected update of "last" value, that's why waiting test doesn't work properly.

It would make more sense for iteration function (callback function) to return a promise instead of accepting a callback argument.
Or, on the other hand, slowLoop could accept a callback instead of returning a promise.

Yes, that's what I mean.

@Pamblam, I think he talks about consistency.
It would make more sense for iteration function (callback function) to return a promise instead of accepting a callback argument.
Or, on the other hand, slowLoop could accept a callback instead of returning a promise.

promises are basically useless without at the very least a "then" callback.

What about async functions?

promises use callbacks. promises are basically useless without at the very least a "then" callback. not sure what your problem is.

You're welcome.

@Chrono79: thanks for all your answers!

Yes, that's why 11 comes before 2000.

Oh, I have passed! It was a bit difficult :) The only thing I don’t understand to is why the second expected test result is "11 11 2000 10003 22 123 1234000 44444444 9999"? Isn’t "2000" bigger than "11" in the case of strings? Similar to "10003 22"?

Returning errors should probably be tested too.

https://www.codewars.com/kata/reviews/5a8cf0cc6e3b30d630000575/groups/5a8d37e46c083ceede00066c
Asynchronous things should be tested with a timeout in describe:
https://www.codewars.com/kumite/59f4abb427402f96e8000051?sel=59f9e16ac374cbf19a00007e

Why even mix callbacks with promises?

It's not fixed yet:

https://www.codewars.com/kata/reviews/5a8cf0cc6e3b30d630000575/groups/5a8d0afc6e3b30c2d9001d26

You still need to fix the message strings of the test assertions.